Method of making hermetically sealed mirrors



UNITED STATES PATENT OFFICE METHOD OF MAKING HERMETICALLY .SEALEDMIRRORS Rowland D. Smith, Corning, N. Y., assignor to Corning GlassWorks, Corning, N. Y., a corporation of New York No Drawing. ApplicationNovember 14, 1933, Serial No; 698,025

4 Claims. (Cl. ill-68.2)

This invention relates to mirrors, and more parafter called silverpaste, comprising, as commonly ticularly to mirrors comprising a thinlayer of known to the art, aflux such as lead borate, finely silver orother highly reflecting metal deposited powdered metallic silver, orsilver compound, and on glass. a vehicle such as lavenderoil. I havefound that 5 In making such mirrors and particularly those such silverpaste fluxes more readily into glass if 5 coated with silver, it hasbeen customary to apply a small amount, say of lead fluoride is incoraprotective coating to the silver layer to prevent porated therewith, andI prefer to use such a mixtarnishing, peeling, or other deteriorationthereof. ture. The silver paste mixture is applied to the Such coatingshave comprised paints, lacquers glass in the desired position with abrush and the 10 and varnishes with or without a sub-layer of copglassis then fired to a temperature of about 600 1 per depositedelectrolytically over the silver. As C. for a few minutes, after whichthe glass is far as I know, all prior coatings or methods of cooled toroom temperature, preferably at such protecting mirrors have beenunsatisfactory in a rate as to anneal it. that they do not permanentlyprotect the mirror I then form a mirror of a suitable reflecting fromdeterioration, due to atmospheric influmetal on the glass within thebordering metal 5 ences. Paints, lacquers, and even electroplated bandso that the mirror slightly but not entirely copper will not permanentlyadhere to glass, espeoverlaps the border. In forming the mirror, Icially under changing temperature and atmosprefer to deposit silverchemically from solution pheric conditions. by means of a reducing agentin accordance with 2c The primary object of this invention is to herthewell known Brashear method of depositing metically seal metal mirrors onto glass so that silver mirrors. For purposes requiring a cheaper theyare permanently impervious to atmospheric and less reflective mirror, Imay use zinc or alumiinfluences and are capable of withstanding all numor alloys thereof applied to the glass by thermal changes incident totheir use. spraying the molten metal as in the Schoop 25, Another objectis to make a mirror, the backing process, or by any other suitablemeans. material of which can expand or contract inde- After the mirroris formed, I make the exposed pendent of the mirror itself. metalsurface the cathode in a copper plating The above and other objects maybe attained by bath comprising preferably a 10% solution of practicingmy invention which embodies among potassium bitartrate saturated withbasic copper its features the method which comprises depositcarbonateand made slightly alkaline with potas- 30 ing a reflecting layer ofmetal, such as silver, on slum carbonate, as described in the Journal,of the glass so as to slightly overlap an encircling the AmericanOptical Society, vol. 25, page 237. border of metal which has previouslybeen fired A current density of about .01 to .02 ampere is into theglass, electroplating the exposed surface applied for sumcient length oftime to secure a a5 and coating the electroplated layer with a suitthinadherent coating of copper covering the able backing material, such assolder or a ceramic deposited metal mirror and the exposed portionglaze. of the metallic band which borders it.

Another feature of my invention comprises in- I then coat the platedcopper coating with a troducing between the mirror and its backingsuitable backing material, such as a low melting no material alayer ofarelatively non-adhesive submetal or metal alloy, for example, solder,or a 40 stance, such as graphite. suitable ceramic glaze. The soldercoating is Although, if precautions are taken to avoid sud best appliedby dipping the article into a bath den thermal changes, I may apply mymirror to of molten solder so as to immerse the portion ordinary highexpansion glass, I prefer to use a which is covered by the copperplating. In apheat resistant glass such as the glass B2 of the plyingthe solder coating, I prefer to use a suit- 5 Sullivan and Taylor PatentNo. 1,304,623, in the able flux, such as rosin, to cause the solder touse of which such precautions are practically wet the copper morereadily. The ceramic glaze, unnecessary, many formulae for which areavailable in the In practicing my invention, I fire into the glassliterature, should have a fusion point below the a narrow band, ,4 to Ain width, of metal softening point of the glass and is applied as a 5gcompletely encircling and bordering the area slip and fired on in knownmanner. which is to be made a mirror. For this purpose I In the mirrorsthus formed, the reflecting may use any of the so called metalliclusters, such metallic surface is completely and hermetically assilver,platinum, gold, and the like, butIprefer covered by the sup p s d a s fpp to use silver luster in the form of a paste, hereinplating andbacking material. The overlapping 5 layers of copper and solder orceramic material are permanently attached to the metallic band whichborders the reflecting metallic surface and which in turn is permanentlyincorporated with V.

the glass surface. It will thus be evident that mirrors made by myprocess are permanently protected from atmospheric influences. v

However, it sometimes happens that the plated copper coating will besomewhat porous and will have one or more minute perforations orpinholes, due to failure of the electrolysis to cover properly. In thiscase the solder coating tends to penetrate through the pores and alloywith the reflective silver coating, thereby impairing its reflectingefllciency. Furthermore, the difference in expansion coeflicient of theglass and the backing'material will sometimes cause the mirror to betorn away from the glass with consequent damage to the reflectingsurface. I have found that I can overcome these difficulties by placingbeneath the copper plating a layer ofsome material which is electricallyconducting, which is not wet by the solder, and which will not resistshearing stress, and hence will prevent penetration of the solder ortearing of the mirror by expansion or contraction of the backingmaterial. Graphite is a good conductor, does not wet with solder noradhere strongly to the mirror or its backing material, and hence isparticularly suitable for this purpose. For best results in applying thegraphite layer, I prefer first to copper plate the silver coatings, asabove described, in order to gain additional protection for the delicatesilver mirror and then to apply the graphite over the copper plating tobe followed by a second plated copper coating over the graphite. Theform of graphite best suited for my purpose is a very finely comminutedgraphite, and I prefer to use the preparation known as Aquadagconsisting of colloidal graphite suspended in water which I apply bydipping or by spraying. In applying the graphite layer, I do not coverthe entire plated coating of copper, but I leave a narrow band aroundthe edge thereof uncovered. In other words, I apply the graphite only tothat portion of the copper plating which covers the reflecting mirror.This is done in order that the graphite layer may be completely sealedbetween the first plated copper coating and the second plated coppercoating which covers the graphite and merges with the exposed edge ofthe first plated copper coating. After the graphite has been applied, Iallow it to dry and become set so that it will not wash of! in thesucceeding operation.

I then plate with copper over the graphite and over the exposed edge ofthe first plated copper coating by means of the electrolytic bathdescribed above. The second copper plating is then covered with solderor a ceramic glaze in the manner described above.

Mirrors thus made with an enclosed graphite 9 layer can be produced withgreat uniformity and with no losses due to imperfections or to thermalexpansion differences.

I What I claim is:

1. The method of hermetically sealinga metal mirror on glass, whichincludes firing a narrow band of metal luster into the glass, completelybordering the area to be covered by the mirror, depositing a metalmirror within the area and slightly overlapping the fired-in border,electroplating the mirror and the border with copper and coating thecopper layer with a backing material.

2. The method of hermetically sealing a metal mirror on glass, whichincludes firing a narrow band of metal luster into the glass completelybordering the area to be coverpd by the mirror, depositing a metalmirror within the area, and slightly overlapping the fired-in border,electroplating the mirror and the border with copper, coating the copperabove the mirror with finely comminuted graphite, electroplating overthe graphite and the border with a second layer of copper and coatingthe second layer of copper with a backing material.

3. The method of hermetically sealing a silver mirror on glass, whichincludes firing into the glass a narrow strip of silver paste to form aband of metallic silver completely bordering the area to be covered bythe mirror, depositing a silver mirror within the area and slightlyoverlapping the fired-in border, electroplating themirror and the borderwith copper and coating the copper layer with solder.

4. The method of hermetically sealing a silver mirror on glass, whichincludes firing into the glass a narrow strip of silver paste to form aband of metallic silver completely bordering the area to be covered bythe mirror, depositing a silver mirror within the area and slightlyoverlapping the fired-in border, electroplating the mirror and theborder with copper, coating the copper above the mirror with finelycomminuted graphite, electroplating over the graphite and the borderwith a second layer of copper, applying. a solder flux to the secondlayer of copper and coating the fluxed surface with molten solder.

ROWLAND D. SMITH.

